NF.sub.3 is a colorless gas having such physical properties that the boiling point is -129.degree. C., and the melting point is -208.degree. C.
Since the activity of NF.sub.3 is suitably weak as a fluorine source in comparison with fluorine (F.sub.2), and the toxicity thereof is low, conventionally NF.sub.3 was used as a fluorine source in adjusting fluoroolefins, and was used as an oxidizing agent of high energy fuels. Also, in recent years, NF.sub.3 as a cleaning agent for CVD apparatuses in the field of electronic materials as well as as a dry etching gas for very large-scale integrated circuits has become to be used.
There are many processes for the production of NF.sub.3. As major processes thereof can be mentioned, for example, a molten salt electrolytic process using an ammonium acid fluoride (U.S. Pat. No. 3,235,474), a process wherein an ammonium acid fluoride in a molten state is reacted with gaseous fluorine (Japanese Patent Publication No. 8926/1980), and a process wherein an ammonium complex of a metal fluoride in the solid state is reacted with F.sub.2 in the elemental state (Japanese Patent Publication No. 21724/1987).
In any of these processes, in the reaction step of producing NF.sub.3, it is required that an inert gas is introduced as a carrier gas into the reaction system in order to prevent explosion to improve the safety or in order to control the reaction suitably, and as this inert gas, inexpensive nitrogen (N.sub.2) gas is employed in most cases. Even in the case wherein the reaction is carried out without making a particular introduction of a carrier gas from the outside, in any of the above processes, N.sub.2 gas is formed as a by-product, and remains mixed in the formed NF.sub.3 gas.
Therefore, when NF.sub.3 produced in any of the above processes is to be used for various applications as mentioned above, it is required to eliminate impurities such as N.sub.2 gas, nitrous oxide (N.sub.3 D), and carbon dioxide (CO.sub.2). However, since generally N.sub.2 gas (N.sub.2 gas as a carrier gas, and N.sub.2 gas as a by-product) is contained in a considerable amount in the NF.sub.3 produced in any of the above processes, in order to eliminate the N.sub.2 gas, the so-called NF.sub.3 condensing step wherein the N.sub.2 gas is eliminated after the reaction step is joined to follow the reaction step. (Note that, in some cases, a purifying step of eliminating impurities other than N.sub.2 gas such as N.sub.2 O and CO.sub.2 as mentioned above, and hydrogen fluoride (HF) that has remained unreacted or has been produced as a by-product is also placed between the reaction step and the condensation step.)
In this case, although there are various ways of condensing NF.sub.3, a process of liquefying NF.sub.3 using a refrigerant is commonly used as the most effective process since impurities will not be introduced and the facilities are simple in comparison with other processes wherein compression by using a compressor is effected. As the refrigerant for liquefying, use is made of a liquefied gas having a boiling point lower than that of NF.sub.3 such as liquid nitrogen, liquid air, and liquid argon, and when, of these, liquid nitrogen is used, it is the most preferable because, for example, the condensation of NF.sub.3 becomes easy, N.sub.2 is an inert substance, therefore the use thereof is safe, and N.sub.2 is inexpensive.
However, in the case wherein NF.sub.3 is produced by using apparatus including the production steps as mentioned above, when the NF.sub.3 formed in the condensation step is cooled and liquefied by using liquid nitrogen as a refrigerant, because N.sub.2 gas that is a carrier gas and N.sub.2 gas that has been formed as a by-product are also partly liquefied together with NF.sub.3, the pressure in the condensation step is decreased, and the pressure in the whole reaction system is also decreased.
Since the formation reaction of NF.sub.3 is carried out continuously, such a state makes quite difficult the control of the reaction in the reaction step, and safety problems arise, making impossible the reaction to be continued for a long period of time.
This is described using the molten salt electrolytic process as an example as follows. For example, in the case wherein, in order to produce NF.sub.3, a molten salt electrolysis is carried out in an NH.sub.4 F/HF system using, as raw material, acid ammonium fluoride or ammonium fluoride and hydrogen fluoride or a KF/NH.sub.4 F/HF system that is formed by adding as a raw material, acid potassium fluoride or potassium fluoride to the NH.sub.4 F/HF system, NF.sub.3 gas is released from the anode of the electrolytic cell while H.sub.2 gas is released from the cathode.
If the NF.sub.3 gas and the H.sub.2 gas are mixed, they will explode, and therefore, in order to avoid the mixing, as shown in FIG. 3, the electrolytic cell is provided with a partition 13 for separating the anode 14 and the cathode 15 in such a manner that the partition 13 is fixed to a lid plate 16 with a suitable distance between it and the bottom plate of the electrolytic cell, so that the electrolytic cell is formed with an anode chamber 18 and a cathode chamber 19 (the partition is positioned in such a way that the electrolyte can move freely between the anode chamber and the cathode chamber at the bottom part of the electrolytic cell.) to prevent the NF.sub.3 gas and the H.sub.2 gas from mixing. In order to prevent the partition at the time of the electrolysis from having a bipolarization phenomenon, the partition is made of a fluorocarbon resin or has the surfaces coated with a fluorocarbon resin.
As the fluorocarbon resin used for the partition in the present invention, any of generally known ones can be suitably used such as polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene/hexafluoropropylene copolymer, tetrafluoroethylene/ethylene copolymer, tetrafluoroethylene/perfluoroalkylvinyl ether copolymer, and chlorotrifluoroethylene/ethylene copolymer.
The NF.sub.3 gas and the H.sub.2 gas released respectively from the anode chamber and the cathode chamber of the electrolytic cell are led to the outside of the system, and then the NF.sub.3 gas is led to the next step, that is, the condensation step while the H.sub.2 gas is utilized as a by-product or is burnt and then released into the atmosphere. Therefore it is impossible to keep the pressure in the anode chamber and the pressure in the cathode chamber at the same level all the time, resulting in a pressure difference. As a result, the level of the electrolyte in the anode chamber becomes different from the level of the electrolyte in the cathode chamber.
As the pressure difference becomes great, the difference in the level of the electrolyte surface becomes greater between the anode chamber and the cathode chamber, and when this difference exceeds a certain limit, either the NF.sub.3 gas or the H.sub.2 gas passes below the partition, and then mixes with the other gas thereby leading in the end to formation of a gas within the explosion limit. In order to obviate this, an inert gas is introduced as a carrier gas into the anode chamber and the cathode chamber of the electrolytic cell respectively in suitable amounts to control the difference. Thus, the term "carrier gas" means a gas that is fed into the reaction tank for the purpose of improving the safety and controlling the reaction.
In this case, as the carrier gas, a gas that is inert to NF.sub.3 gas and H.sub.2 gas, and can be easily separated from the NF.sub.3 gas in the subsequent purifying step for example N.sub.2, or Ar is used, and since, of these gases, N.sub.2 gas is inexpensive, it is preferable. Further, in the molten salt electrolytic process, since N.sub.2 is formed as a by-product in a considerable amount in the reaction step, and the thus formed N.sub.2 interfuses into the produced NF.sub.3, it is also preferable to use N.sub.2 gas as the carrier gas.
The NF.sub.3 gas continuously produced in the electrolytic cell is then cooled with liquid nitrogen as stated above to be liquefied and condensed. (It should be noted that, in the case of the molten salt electrolytic process, since for example hydrogen fluoride, etc. are included as unreacted material, it is preferable that a purifying step of eliminating the hydrogen fluoride, etc. is placed between the reaction step and the liquefying/condensing step.)
However, since, in this liquefying/condensing step, the N.sub.2 gas introduced as a carrier gas into the electrolytic cell in the reaction step is also partly liquefied, the pressure in the liquefying/condensing apparatus decreases, and due to this the pressure in the anode chamber of the electrolytic cell also decreases, leading to a pressure difference between the anode chamber and the cathode chamber. As a result, for the same reason as stated before, the hydrogen gas will mix into the NF.sub.3 gas formed in the anode chamber thereby forming an explosive gas.
Since it is impossible to prevent completely the drop of the pressure in the anode chamber that will lead to the formation of the explosive gas only by the control of the supply of N.sub.2 gas into the anode chamber, there is a serious problem that the operation cannot be continued for a long period of time to produce NF.sub.3.
Therefore, it is desired to develop a method that can solve the above-mentioned problems, is safe, and can produce NF.sub.3 continuously for a long period of time, and more specifically to develop a method of recovering NF.sub.3 by liquefaction and condensation from a mixed gas of NF.sub.3 obtained by a molten salt electrolytic process and a carrier gas.
NF.sub.3 is produced by the various processes as mentioned above, and generally the thus produced NF.sub.3 contains relatively large amounts of impurities such as nitrous oxide (N.sub.2 O), carbon dioxide (CO.sub.2), oxygen (O.sub.2), and nitrogen (N.sub.2) Therefore, for example, when the NF.sub.3 is to suitably be used as a cleaning agent for CVD apparatuses in the field of electronic materials or as a dry etching agent or the like in the production of LSI's, it is required that the above-mentioned impurities are eliminated as far as possible to make the NF.sub.3 highly pure.
As a method of eliminating the above-mentioned impurities, usually a method is used wherein the NF.sub.3 is brought in contact with an adsorbent such as zeolites thereby eliminating the impurities, because the method is the most effective and simple. However, although the treatment with a zeolite can eliminate efficiently impurities having relatively high boiling points such as N.sub.2 O, and CO.sub.2 present in the NF.sub.3, the treatment can hardly eliminate components having relatively low boiling points such as O.sub.2, and N.sub.2, and a method of eliminating them has not yet been known.
Although distillation technique is effective and is commonly used generally as means of separating components having different boiling points, when the inventors attempted distillation technique to eliminate low-boiling components in the NF.sub.3, the low-boiling components could not efficiently be separated in spite of the fact that there is a sufficient difference in the boiling point between NF.sub.3 and the low-boiling components, and it was found that for example N.sub.2 remained in an amount of at least as much as thousands ppm in the NF.sub.3.
Development of a method of eliminating efficiently low-boiling components such as O.sub.2, and N.sub.2 to yield highly pure nitrogen fluoride is desired.